Catalyst components of the Ziegler-Natta type for the stereospecific polymerization of olefins are widely known in the art. The latest developed catalysts for propylene polymerization comprise a solid catalyst component, constituted by a magnesium dihalide on which are supported a titanium compound and an internal electron donor compound, used in combination with an Al-alkyl compound and often with an external donor which is needed in order to obtain higher isotacticity. One of the preferred classes of internal donors is constituted by the esters of aromatic carboxylic acids such as benzoates and phthalates this latter being the most used in view of their excellent performances. The phthalates are used as internal donors in combination with alkylalkoxysilanes as external donor. This catalyst system is capable of giving good performances in terms of activity, and propylene polymers with high isotacticity and xylene insolubility endowed with an intermediate molecular weight distribution.
As the use of some phthalates has been recently addressed as involving potential toxicity problems, the interest in exploration of alternative classes of donor has increased.
U.S. Pat. No. 5,041,403 discloses alkoxyesters of formula (R1O)i(R2O)j(R3O)k—Z—COOR4 (I) wherein R1, R2, R3 and R4 are an aliphatic, alicyclic, aromatic, polycyclic or heterocyclic compound group, Z is an aliphatic or alicyclic hydrocarbon group in which hydrogen may be substituted with an aromatic or polycyclic group, and i, j and k are integers of 0 to 3 with the proviso that the sum of i, j and k is at least 1. Among this broad class, the 3-alkoxy propionate derivatives substituted in 2-position with bulky, preferably aromatic, rings are those more preferred. Their performances however, in terms of activity/stereospecificity balance seem not fully satisfactory.